Aequorin

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Aequorin ribbon diagram from PDB database

Aequorin is a photoprotein isolated from luminescent jellyfish (like various Aequorea species e.g. Aequorea victoria) and a variety of other marine organisms.

Aequorin is composed of two distinct units, the apoprotein apoaequorin, with an approximate molecular weight of 22 kDa, and the prosthetic group coelenterazine (molecular weight 472), a molecule belonging to the luciferin family.

In the presence of molecular oxygen the two components of aequorin reconstitute spontaneously, forming the functional protein. Researchers have located a number of EF-hand type regions in the structure of Aequorin that function as binding sites for Ca²⁺ ions: when Ca²⁺ occupies such sites, the protein undergoes a conformational change and converts through oxidation its prosthetic group, coelenterazine, into excited coelenteramide and CO₂. As the excited coelenteramide relaxes to the ground state, blue light (wavelength = 469 nm) is emitted.

Since the emitted light can be easily detected with a luminometer, aequorin has become a useful tool in molecular biology for the measurement of intracellular Ca²⁺ levels. Cultured cells expressing the aequorin gene can effectively synthesize aequorin: however recombinant expression only yields the apoprotein, therefore it is necessary to add coelenterazine into the culture medium of the cells to obtain a functional protein and thus use its blue light emission to measure Ca²⁺ concentration. Coelenterazine is a hydrophobic molecule, and therefore is easily taken up across plant and fungal cell walls, as well as the plasma membrane of higher eukaryotes, making aequorin suitable as a Ca²⁺ reporter (calcium in biology) in plants, fungi and mammalian cells.

Aequorin has a number of advantages over other Ca²⁺ indicators: it has a low leakage rate from cells, lacks phenomena of intracellular compartmentalization or sequestration and it does not disrupt cell functions or embryo development. Moreover the light emitted by the oxidation of coelenterazine does not depend on any optical excitation, so problems with auto-fluorescence are eliminated.